3.2 Chromosomes Flashcards
Electron migrograph of Escherichia coli (E. coli) bacteria.
What are prokaryotes?
-Microscopic, single-celled organisms without specialised compartments inside the cell.
-Prokaryotes include two of the three domains of life: eubacteria and archaea.
What is a genome?
The entire sum of an organism’s genetic information.
Describe the genome in prokaryotes
In prokaryotes, there is usually a single chromosome that contains the entire genome, including all the genes necessary to carry out the functions of life.
Describe the structure of prokaryotic chromosomes
-The chromosome is a circular DNA double helix molecule.
-The chromosome has the shape of a closed loop, like a rubber band.
-Prokaryotic DNA is called ‘naked DNA’ because, in contrast to eukaryotic DNA, it is not organised around associated histone proteins.
What do prokaryotes have in addition to the single chromosome that carries all the essential genes of the species?
Much smaller loops of DNA known as plasmids.
Describe the plasmids in prokaryotes
-A bacterium may contain one or more copies of a plasmid or none at all.
-A plasmid may contain one or several genes, often related to a specific function such as digesting a particular food source or resisting an otherwise deadly antibiotic.
-Bacteria can easily exchange plasmids, sometimes even with a different species of prokaryote.
Describe the presence of plasmids in eukaryotes
-Although plasmids are very common in prokaryotes, they are generally not found in eukaryotes.
-(However, there are exceptions. For example, Saccharomyces cerevisiae, a unicellular yeast used in baking, contains a small plasmid.)
Diagram of a bacterium, showing the plasmid.
The plasmid carries two genes, making the bacterial cell resistant to ampicillin and tetracycline, two commonly used antibiotics.
How can plasmids be exchanged between bacteria?
-Plasmids can be easily exchanged between bacteria, providing a mechanism by which antibiotic resistance spreads.
-Antibiotic-resistant bacteria are a major problem in hospitals.
Diagram of a plasmid containing two antibiotic resistance genes
What are plasmids?
Much smaller circular DNA molecules that are easily exchanged between prokaryotes and may contain several genes.
Why is the single, circular prokaryotic chromosome referred to as ‘naked’?
Because it is not associated with any proteins.
How do eukaryotic organisms separate their genetic material from the cytoplasm?
By containing it in the membrane-bound nucleus.
Describe eukaryotic chromosomes and DNA
-Eukaryotic chromosomes consist of one or two long linear DNA molecules, depending on the phase of the cell cycle .
-Each DNA molecule has two ends, like a piece of thread.
-The DNA molecule is wrapped around basic (alkaline) proteins called histones.
Diagram of the eukaryotic chromosome structure: DNA and histones form nucleosomes
How are eukaryotic chromosomes present during interphase?
During interphase, when the information on the DNA is in active use, the chromosomes are visible only with an electron microscope in a less organised form called chromatin.
What does DNA being organised as chromatin during interphase allow?
This allows the DNA-transcribing enzymes to access the genes with the information they need to keep the cell functioning.
Explain what happens to chromosomes in the first stages of mitosis (and meiosis)
The chromosomes condense to form highly organised ‘X-shaped’ structures visible with a light microscope.
Describe the structure of X-shaped chromosomes (including sister chromatids and the centromere)
-The two lines that form the ‘X’ are identical copies of the chromosome, known as sister chromatids.
-The place where the two strands connect is called the centromere .
What is the purpose of the centromere?
It holds the sister chromatids of a chromosome together until they are separated in anaphase (at which point each chromatid is considered a chromosome).
What does the supercoiled state of a chromosome allow?
It allows them to separate without getting tangled and torn.
Describe the chromosomes in most of your body cells
Most of your body cells contain two full sets of chromosomes: one from your mother (maternal copy) and one from your father (paternal copy).
What are homologous pairs and chromosomes?
-Your two versions of the same chromosome are called a homologous pair.
-Homologous chromosomes carry the same set of genes in the same locations.
-However, there can be small differences between the nucleotide sequences of the same gene.
What are alleles?
-Different versions of the same gene are called alleles.
-Alleles usually differ from each other by only a few DNA base pairs, but may lead to different characteristics and phenotypes.
Diagram of two supercoiled chromosomes, each consisting of two chromatids
What are eukaryotic chromosomes?
Linear DNA molecules associated with histone proteins.
In any eukaryote species, there are different chromosomes that carry ___
Different genes.
Homologous chromosomes carry the same sequence of genes but not necessarily ___
The same alleles of those genes.
How is the human genome arranged into chromosomes?
The human genome is arranged into 23 pairs of chromosomes; that is 46 chromosomes in total.
Explain how sex chromosomes work in humans and other eukaryotes
-One pair of chromosomes, called the sex chromosomes, determines the sex of the organism.
-In species like humans where the male has two different sex chromosomes, the sex chromosomes are referred to as ‘X’ and ‘Y’.
-Usually, if you have two X chromosomes, you develop as a female, while if you have an X and a Y chromosome, you develop as a male.
-Other eukaryotes have different methods of determining the sex of offspring.
What are autosomes?
-The chromosomes in the other 22 pairs (other than the sex chromosomes) are called autosomes, meaning they do not influence sex determination.
-They are numbered roughly according to length.
-Chromosome 1 is the longest and chromosome 22 is nearly the shortest.
-Chromosome 1 is approximately 250 million bases in length and carries about 2 000 genes while chromosome 22 is only 51 million bases and carries around 500 genes.
Different chromosomes carry different ___
Sets of genes
In eukaryotes, the genome consists of the sum of all the ___
Genetic information on all of the different chromosomes.
Table showing the variation in genome size that exists among different organisms
Comparing genome sizes for the exam
You should be able to compare the genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
Graph showing genome size and number of genes in prokaryotes, eukaryotes and viruses.
Notice that although there is a general correlation between genome size and number of genes, there are many outliers.
Diagram of homologous chromosomes
Table whoswing the diploid chromosome number of some species
Picture of a normal female karyogram
Describe the karyogram
-This is a karyogram of a female because two X chromosomes are present.
-The set of autosomes is normal, and there are no duplications or missing chromosomes.
-These chromosomes may appear to be unreplicated, but each actually has two chromatids.
-The sister chromatids are so closely aligned that it is difficult to distinguish them, but they are present.
Diagram showing karyograms for a typical corn plant and a dog
What can be concluded on the basis of the following karyogram?
Male with a normal set of chromosomes
What can be concluded on the basis of the following karyogram?
Female with Down syndrome
Iillustration of an autoradiograph produced by Cairns’ technique and its interpretation
